Aqueous acrylic and polyurethane dispersions have been widely used in coating applications, both independently and in physical mixtures. By combining both components, the resulting coating can show a balance of properties from the individual inherent characteristics of acrylic and polyurethane products, and the mixture can benefit from the better features of polyurethane and from the reduced cost of acrylic. This can be interesting in the case of outdoor coatings such as coatings for roofs.
However, the properties of these mixtures often do not achieve the aforementioned properties from the traditional “rule of mixtures”. Though the reasons for these unwanted effects in physical mixtures have not yet been well defined, the non-homogeneity caused by the different acrylic and polyurethane domains can contribute to these effects. These areas can have excessive internal stress and/or incomplete coalescence, which can cause an increased permeability and reduced bond strength.
For example, U.S. Patent Application Publication No. 2001/0007711 A1 describes a method for coating concrete molds comprising the application of at least one plastic mineral composition which can bond to cement, comprising at least one aqueous polymer dispersion containing a percentage not exceeding 2% by weight of copolymerized monoethylenically unsaturated monomers.
International Publication No. WO 96/00259 describes a method for producing a thermosetting elastomeric matrix, comprising vulcanizing an elastomeric material and a thermoplastic polymer material having a predetermined softening/melting temperature, producing a dispersion of the thermoplastic polymer material through the vulcanizable elastomeric material, and finally curing the thermoplastic-containing matrix.
U.S. Patent Application Publication No. 2005/0124736 A1 describes aqueous polymer compositions containing at least one aqueous bitumen dispersion and at least one aqueous dispersion of at least one polyurethane. This composition is used in surface dressings and as watertight roof coatings.
U.S. Pat. No. 4,331,726 describes pourable roofing compositions comprising discrete small chunks or pieces of closed cell plastics material, such as polyurethane, an inert particulate material, and an emulsion caulking or sealing type binder or adhesive based on acetates, acrylic resins, epoxy adhesives, and the like.